1. Field of the Invention
The present invention relates to an apparatus and method for luminance control in a liquid crystal display device, and more particularly, to an apparatus and method for luminance control in a liquid crystal display device capable of reducing power consumption and improving picture quality by selectively driving a lamp.
2. Description of the Related Art
The scope of application for liquid crystal displays (LCDs) continues to increase due to the advantages of LCDs, such as lightness, thinness, and low power consumption. Generally, LCDs are used in office automation devices, audio/video devices and the like. An LCD adjusts the transmittance of light beams in accordance with an image signal to display desired pictures on a screen.
Since the LCD is not a spontaneous light-emitting display device, the LCD device uses a back light unit as a light source. There are two types of back light units utilized in LCDs, a direct-below-type, and a light guide plate-type. In the direct-below-type, several lamps are arranged in the plane, and, a diffusion panel is installed between the lamp and the liquid crystal display panel to fix the distance between the liquid crystal display panel and the lamps. In the light guide plate-type, a lamp is installed in an outer part of the LCD device, and a transparent light guide plate provides/guides light such that it is incident to the whole surface of the liquid crystal display panel.
FIGS. 1 and 2 illustrate a LCD with a direct-below-type backlight according to the related art. Referring to FIG. 2, the device includes a liquid crystal display panel 2 to display a picture, and a direct-below-type backlight unit to irradiate uniform light onto the liquid crystal display panel 2.
In the liquid crystal display panel 2, liquid crystal cells (not shown) are arranged between an upper substrate and a lower substrate. In an active matrix type display panel, a common electrode and pixel electrodes are provided. Generally, the pixel electrodes (not shown) are formed on the lower substrate, also referred to as a thin film transistor substrate, for each liquid crystal cell, and the common electrode (not shown) is integrated with the front surface of the upper substrate. Each of the pixel electrodes are connected to a thin film transistor that is used as a switching device. The pixel electrodes along with the common electrode drive the liquid crystal panel in accordance with a data signal supplied through the thin film transistor, thereby displaying pictures corresponding to a video signal.
The direct-below-type backlight unit includes a plurality of lamps 36 arranged parallel to each other; a lamp housing 34 located at the lower part of the lamps 36, a diffusion plate 12 covering the lamp housing 34, and optical sheets 10 located on the diffusion plate 12.
Each of the lamps 36 includes a glass tube filled with an inert gas, and a cathode and an anode installed at opposite ends of the glass tube. The inside of the glass tube is charged with the inert gas, and the phosphorus is spread over the inner wall of the glass tube.
In each of the lamps 36, if an alternating current AC waveform of high voltage is applied to a high voltage electrode and a low voltage electrode from an inverter (not shown), electrons are emitted from the low voltage electrode L to collide with the inert gas inside the glass tube, thus the amount of electrons are increased in geometrical progression. The increased electrons cause electric current to flow in the inside of the glass tube, so that the inert gas is excited by the electron to emit ultraviolet ray. The ultraviolet rays collide with luminous phosphorus spread over the inner wall of the glass tube emitting visible light rays. The high voltage AC waveform is continuously supplied to the lamps 36, therefore the lamps are always turned-on.
The lamp housing 34 prevents leakage of the visible light rays emitted from each of the lamps 36 and reflects light rays, progressing to the side and the rear surfaces of the lamps 36, to the front surface, i.e., toward the diffusion plate 12, thereby improving the efficiency of the light generated at the lamps 36.
The diffusion plate 12 directs the light emitted from the lamps 36 towards the liquid crystal display panel 2 and to be incident in an angle of a wide range. This is achieved, for example, by coating the diffusion plate 12 on both sides with a transparent resin.
The optical sheets 10 narrow the viewing angle of the light emitted from the diffusion plate 12, to improve the front brightness and reduce power consumption in the liquid crystal display device.
A reflection sheet 14 is arranged between the lamps 36 and the lamp housing 34 to reflect the light generated from the lamps 36 so as to direct it towards the liquid crystal display panel 2, thereby improving the efficiency of light.
The related art LCD generates a uniform light by use of the lamps 36 arranged in the lamp housing 34 to irradiate it to the liquid crystal display panel 2, thereby displaying the desired picture. However, the lamps of the related art LCD are continuously turned-on, resulting in high power consumption and the inability to realize peak brightness, wherein peak brightness is that a designated part on the liquid crystal display panel 2 is instantly brightened in order to display a picture like an explosion or a flash on the liquid crystal display panel 2.